Fatbergs, the congealed mass of fat and discarded items that are increasingly blocking Britain’s sewers, are the consequence of the plastic crisis in Britain and contain potentially deadly antibiotic-resistant bacteria, tests show.

A study by Channel 4 in conjunction with Thames Water has analysed the contents of one supersize fatberg discovered underneath the streets of South Bank in central London.

The forensic analysis revealed that cooking fat is the biggest contributor to the crisis, making up nearly 90% of the sample. It also showed a higher concentration of prohibited gym supplements than street drugs such as cocaine and MDMA.

Fatbergs are part of a growing urban problem across the UK as the sewage infrastructure struggles to cope with the population’s changing habits.

One of the troubling findings on Fatberg Autopsy: Secrets of the Sewers, which airs on Tuesday night, was potentially infectious bacteria including listeria, campylobacter and E coli.

According to experts, the bacteria not only poses an immediate risk to operatives who work in sewers, but also to the public in the event of a blockage, as contents of the sewers could come back up through domestic or commercial pipes causing flooding to homes and businesses.

“We and other water companies are facing a constant battle to keep the nation’s sewers free from fatbergs and other blockages,” Thames Water’s waste networks manager, Alex Saunders, said.

Typical items found in the fatberg included condoms, sanitary towels, nappies, cotton buds and wet wipes.

“The fat sticks to the side of the pipe, the wet wipes come down and stick to the fat, other fat comes down and sticks to the wet wipes and that adds to the mass of the fatberg,” Andy Drinkwater, a civil engineering consultant, said.

The tests showed some wet wipes, including some brands labelled as flushable, are unable to disintegrate in a sewer.

The fatberg autopsy also uncovered evidence of Britons’ contact with street and pharmaceutical drugs, including small plastic “baggies”, a needle and syringe, as well as a high proportion of chemicals found in topical creams for acne and paracetamol.

The tests also discovered evidence of hordenine and ostarine, which can be found in performance-enhancing sports supplements. Ostarine, used for muscle gain, is on the World Anti-Doping Agency’s prohibited list and is not licensed for medical use in the UK.

Hordenine and ostarine represented more than half the proportion of pharmaceuticals found in the sample.

The battle to remove fatbergs costs an estimated £80m a year, which is paid by utility customers via their water bills.

A chunk of the monster Whitechapel fatberg is now a superstar museum exhibit. It shines a horrifying light on our throwaway age – but will it stop people clogging up the sewers with the grease from their Sunday lunch?